1. Field of the Invention
The present invention is generally related to an edge triggered apparatus and method for measuring the strain of Bragg gratings written into an optical fiber.
2. Problem to be Solved
It is often desirable to measure optical fibers for strains that occur when the optical fibers are stressed. Such stress can occur if the optical fiber is subject to physical forces that stretch, contort or contract the optical fiber. Stress to the optical fibers can also occur as a result of variations in temperature of the environment within which the optical fiber is located. Such physical forces and temperature variation are typically found in aerospace environments such as aircraft or spacecraft.
Fiber Bragg Gratings have been used to sense strain in optical fibers. Fiber Bragg Gratings comprise a portion of the optical fiber where the index of refraction has been changed. The gratings are written on a section of the optical fiber which is then bonded to longer lead-in, lead-out optical fibers. Coherent light of a specific wavelength is transmitted down the core of the optical fiber. The coherent light reflects off the Bragg Gratings of the same wavelength and passes back up the fiber. As the grating spacing changes in response to strain, the index of refraction of the grating changes thereby altering the period of the modulation of the index of refraction. Multiple Fiber Bragg Grating configurations can also be used for measuring strain. In such a configuration, each Bragg grating has a unique central frequency (and no overlap of the frequency response). Thus, multiplexed signals can be transmitted by the optical fiber and discriminated by the Fiber Bragg Gratings. Conventional systems and methods of interrogating Bragg gratings involve the determination of the center frequency of the grating. These methods typically discard the information available from the entire spectral response of the Bragg grating. Thus, these conventional systems and methods measure only the point strain in an optical fiber.
U.S. Pat. No. 5,798,521 discloses and claims a system and method for measuring the strain at each point in the Bragg grating. In that system, optical radiation is transmitted over one or more contiguous predetermined wavelength ranges into a reference optical fiber network and an optical fiber network under test to produce a plurality of reference interference fringes and measurement interference fringes, respectively. The fringes from the reference cavity are digitized in the same manner as the measurement fringes. After being digitized, a computer algorithm is used to determine the phase of the signal at every point in the data set and, from this phase, the change in wavelength of the tunable laser is determined. A simplified approach to that disclosed and claimed in U.S. Pat. No. 5,798,521 for correcting for nonlinearities in the laser wavelength sweep is desired.
It is therefore an object of the present invention to provide a new and improved apparatus and method for measuring the modulation of the index of refraction of a Bragg grating.
It is another object of the present invention to provide a new and improved apparatus and method for measuring the strain at every point along a Bragg grating.
It is a further object of the present invention to provide a new and improved apparatus and method for measuring the strain at every point along a Bragg grating with a relatively high degree of accuracy.
It is another object of the present invention to provide a new and improved apparatus and method for measuring the strain at every point along a Bragg grating that may be implemented cost effectively.
It is yet another object of the present invention to provide a new and improved apparatus and method for measuring the strain at every point along a Bragg grating that corrects for nonlinearities in the laser wavelength sweep with greater computational efficiency.
It is yet another object of the present invention to provide a new and improves apparatus and method for measuring the strain at every point along a Bragg grating that uses fringes from a reference cavity to trigger the sampling of measurement fringes.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to an apparatus and method for measuring the complete spectral response of Fiber Bragg Gratings so that the strain at each point in the grating can be measured. In accordance with the present invention, optical radiation is transmitted over one or more contiguous predetermined wavelength ranges into a reference optical fiber and an optical fiber under test to produce reference and measurement interference fringes. The optical fiber under test has a plurality of gratings written therein. The reference and measurement fringes are detected, and the reference fringes trigger the sampling of the measurement fringes. This results in the measurement fringes being sampled at 2xcfx80 increments of the reference fringes. Each sampled measurement interference fringe is transformed into a spatial domain waveform such that each spatial domain waveform corresponds to one of the contiguous predetermined wavelength ranges. The spatial domain waveforms are then summed to form a summation spatial domain waveform. The summation spatial domain waveform is then analyzed to determine the location of each grating with respect to a reflector of the optical fiber under test. The next step entails determining from each spatial waveform a portion of the spatial domain waveform that corresponds to a particular grating. Each of these determined portions of the spatial domain waveforms is transformed into a corresponding frequency spectrum representation. The strain on the grating at each wavelength of optical radiation is then determined by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement.